Revisiting the Electrochemical Nitrogen Reduction on Molybdenum and Iron Carbides: Promising Catalysts or False Positives?

The electrochemical dinitrogen reduction reaction (NRR) has recently gained much interest as it can potentially produce ammonia from renewable intermittent electricity and replace the Haber–Bosch process. Previous literature studies report Fe- and Mo-carbides as promising electrocatalysts for the NRR with activities higher than other metals. However, recent understanding of extraneous ammonia and nitrogen oxide contaminations have challenged previously published results. Here, we critically assess the NRR performance of several Fe- and Mo-carbides reported as promising by implementing a strict experimental protocol to minimize the effect of impurities. The successful synthesis of α-Mo2C decorated carbon nanosheets, α-Mo2C nanoparticles, θ-Fe3C nanoparticles, and χ-Fe5C2 nanoparticles was confirmed by X-ray diffraction, scanning and transmission electron microscopy, and X-ray photoelectron and Mössbauer spectroscopy. After performing NRR chronoamperometric tests with the synthesized materials, the ammonia concentrations varied between 37 and 124 ppb and are in close proximity with the estimated ammonia background level. Notwithstanding the impracticality of these extremely low ammonia yields, the observed ammonia did not originate from the electrochemical nitrogen reduction but from unavoidable extraneous ammonia and NOx impurities. These findings are in contradiction with earlier literature studies and show that these carbide materials are not active for the NRR under the employed conditions. This further emphasizes the importance of a strict protocol in order to distinguish between a promising NRR catalyst and a false positive.


INTRODUCTION
The activation of diatomic nitrogen has been one of the greatest challenges in nitrogen-related chemistry. 1,2 This is inherently related to the inert nature of the nitrogen molecule due to its high bond-dissociation energy, absence of a dipole moment, and low proton and electron affinity. 3 Despite the inert nature, diazotrophic microorganisms successfully fixate nitrogen and play a key role in enriching the soil. 4 However, due to the growing world population and the high demand for food, additional nitrogen containing nutrients in the form of artificial ammonia-based fertilizers must be provided to the soil. The majority of the ammonia produced worldwide is synthesized by the Haber−Bosch process (H-B), wherein energy intensive reaction conditions (T = 300−500°C, P = 150−300 bar) are needed to activate dinitrogen. 2 The ammonia industry consumes approximately 1% of the global energy demand and emits roughly 0.75% of the anthropogenic CO 2 emissions (assuming 1.9 t COd 2 t NHd 3 −1 ), 5,6 which is motivating the search for more energy efficient and sustainable alternatives.
The electrochemical nitrogen reduction reaction (NRR), wherein dinitrogen, water, and electrons from renewable sources react to form ammonia, has recently gained significant scientific interest and has been proposed as a potential replacement for the fossil fuel-based H-B. 7,8 NRR systems at high (>500°C) and intermediate (100−500°C) temperatures have proven to be successful in terms of faradaic efficiency (FE ≥ 75%) and NH 3 yield (≥4.5 nmol s −1 cm −2 ). 9 Nevertheless, the present high temperature NRR systems tend to have a low energy efficiency compared to H-B. 10,11 Therefore, it would be beneficial to perform the NRR under ambient conditions. Significant FEs have been reported with iron electrocatalysts in ionic liquids and organic electrolytes by suppressing the parasitic hydrogen evolution reaction (HER). 12,13 However, the use of organic electrolytes is likely to be less economically feasible compared to aqueous electrolytes due to complex scalability, safety issues, high costs, and intense energy requirements. 8,14 Unfortunately, the kinetics under ambient conditions in aqueous electrolytes are sluggish and many attempts in examining transition metals, such as Au, Fe, Mo, Ru, Rh, and Re, as potential electrocatalysts for the NRR have resulted in low FEs (<1%) and ammonia yields (<0.1 nmol s −1 cm −2 ). 15,16 The active site of the nitrogenase enzyme, the biologic pathway for nitrogen fixation, is the FeMo-cofactor. The FeMo-cofactor contains a six iron atomic trigonal prism with a carbon-centered position. Each iron is bound to three sulfur atoms, with an additional iron and molybdenum in apical positions. 2,17 Attempts to mimic the FeMo-cofactor initiated investigation into Fe-and Mo-based heterogeneous NRR catalysts, such as carbides and sulfides. Both Mo 2 C and MoS 2 show noble metal like properties, due to similar d-band configurations as Pt. 18 Therefore, they can act as cheap and robust catalytic substitutes for many applications, including water electrolysis, water gas shift reaction, and ammonia decomposition. 19 Despite the fact that these materials are good HER catalysts, several density functional theory studies have predicted favorable nitrogen binding energies. 19−21 Experimental results are somewhat distinct; nitrogen reduction experiments with amorphous 2H-MoS 2 and metallic 1 T-MoS 2 did not produce quantities of ammonia exceeding the background level, 12,21 while other studies report reasonable FEs and ammonia yields using FeS 2 , Mo 2 C, and Fe 3 C, thereby labeling these materials as promising NRR catalysts. 22 −24 The electrochemical NRR field is plagued by questionable results, mainly due to the large impact of extraneous ammonia sources on experiments performed on a small scale. Ammonia stemming for other sources can erroneously be assigned to ammonia synthesized by the NRR, which can lead to false positives. Ammonia impurities can be minimized by a proper experiment design and can be identified by applying the right control experiments, such as argon and open-circuit blank tests and ultimately 15 N 2 -labeled experiments. Recently, nitrogen oxides have been identified as another source of contamination, as these species are more easily reduced to ammonia than dinitrogen in the NRR. 25 The majority of the recently published studies have applied blank tests, but performing quantitative 15 N 2 -labeled experiments and monitoring nitrogen oxide species are done sporadically. 25 As a consequence, a handful of research groups have tried to reproduce electrocatalysts initially labeled as promising, such as Fe, Bi, Au, VN, CoMo, Mo 2 N, and MoS 2 , 16,21,26−30 but discovered that the quantified ammonia must originate from sources other than the NRR. Here, we critically assess the electrocatalytic NRR activity of molybdenum and iron carbides, where more than 10 independent literature reports claim to observe superior or excellent catalytic performance. 23,24,31−38 In the present work, α-Mo 2 C nanodots from Cheng et al. (reported as the most promising carbide catalyst) are reproduced and compared with α-Mo 2 C nanoparticles as a benchmark. 23 Additionally, nanostructured θ-Fe 3 C and χ-Fe 5 C 2 are synthesized and examined for their NRR activity. A key aspect of this work is the implementation of a strict protocol, which is designed to minimize the level of extraneous contamination, 15,25 allowing genuine quantification of ammonia produced by the NRR. Mo 2 C nanodots (Mo 2 C NS) were synthesized by a moltensalt synthesis procedure as reported in detail elsewhere. 23 In short, a mixture of 1 mL of water and 4 mL of ethanol (96%, VWR) was mixed in a beaker and continuously heated and stirred on a hotplate. Once the mixture reached 70°C, 0.4 g of bis(acetylacetonato)dioxomolybdenum(VI) and 0.14 g of sucrose (99.5%) were added. After the mixture turned green, an excess amount of sodium chloride (99.5%) was added until a green crystalline slurry was formed. The slurry was directly transferred to a ceramic boat and placed inside a tubular furnace (Blue, Lenton), where the specimen was heated to its carburization temperature under an Ar atmosphere (T carb = 800°C, heating rate = 5°C min −1 ), kept constant at this temperature for 2 h and the furnace cooled down to room temperature naturally. The resulting black catalyst/salt mixture was excessively rinsed with ultrapure water to remove the sodium chloride. The residue was filtrated (Durapore 100 nm, Merck) and dried in an oven at 80°C overnight.
2.2.2. α-Mo 2 C Nanoparticles. Goḿez-Marín and Ticianelli reported a procedure for the synthesis of porous Mo 2 C nanoparticles (Mo 2 C NP) that was replicated here. 39 In a typical procedure, 0.15 g of Vulcan VC-72 (Cabot) was mixed with 0.51 g of MoO 3 (99.9%) in a beaker containing 30 mL of ethanol. The dispersion was heated to 60°C overnight while continuously stirring to evaporate the ethanol completely. The powder was transferred to a ceramic boat for carburization inside a tubular furnace under 10 vol % H 2 :Ar. The precursor was heated to 725°C for 30 min with a slow heating rate (1°C min −1 ) and cooled down to room temperature.

Mesoporous Fe 3 C.
A combined hard-templating and carburization method developed by Kraupner and coworkers was used to create a mesoporous Fe 3 C structure with a high surface area. 40 In brief, 0.5 g of FeCl 3 (99.9%) was dissolved in 1 g of 40 wt % SiO 2 in H 2 O (Ludox AS40) in a borosilicate test tube. Additionally, 0.728 g of 4.5-dicyanoimidazole (99%) was added and stirred through the suspension and sonicated for 30 min to achieve a homogeneous yellow-colored thick slurry paste. The paste was transferred to a ceramic boat and carburized inside a tubular furnace at 700°C (heating rate 2°C min −1 ) for 2 h under an Ar atmosphere and cooled down to room temperature.
2.2.4. χ-Fe 5 C 2 Nanoparticles. The principle of thermodecomposition of Fe(CO) 5 is a common used strategy to synthesize iron carbides and is discussed in detail elsewhere. 41 A mixture of 0.2 g of polyvinylpiyrrolidone (PVP, 40000 g/ mol) and 1 mL of Fe(CO) 5 (99.99%) was inserted in a homemade air-tight reactor consisting of Swagelock tubes and adapters ( Figure S1). The reactor was purged with Ar at a flowrate of 20 mL min −1 for approximately 10 min to remove residual oxygen, and immediately afterward, all Swagelock adapters were closed. The reactor was positioned inside a muffle furnace programmed with T carb at 300°C (heating rate 2.3°C min −1 ) for a duration of 24 h.

Characterization. 2.3.1. X-ray Diffraction (XRD).
Samples were deposited on a Si510 zero background wafer and positioned inside a Bruker D8 Advance diffractometer in Bragg−Brentano geometry equipped with a graphite monochromator, a Vantec position sensitive detector, a variable divergence slit, and a 5 mm height scatter screen. Co Kα radiation (λ = 0.1789 nm) was used to avoid incident beam fluorescence effects on the Fe carbides. During each acquisition, steps with a size of 0.038°and 5 s per step were measured over a 10−110°2θ range. Bruker DiffracSuite.EVA v6.0 was used to subtract the background, correct small displacements, and strip the Kα2 contribution from the patterns to enable crystallite size (D XRD ) estimation with the Scherrer equation (eq 1), where λ is the wavelength, and κ the shape factor taken as 1. Peak shapes were assumed Gaussian, and the full width at half-maximum, in this case β, was additionally corrected for instrumental line broadening effects.

Mossbauer Spectroscopy. Transmission 57 Fe
Mossbauer spectra were collected at room temperature with a conventional constant-acceleration spectrometer with a 57 Co-(Rh) source. Velocity calibration was carried out using an α-Fe foil. The Mossbauer spectra were fitted using the Mosswinn 4.0 program. 42

X-ray Photoelectron Spectroscopy (XPS).
A Thermo Scientific Kα spectrometer with a monochromatic Al Kα excitation source was used to acquire X-ray photoemission spectroscopy (XPS) spectra. The base pressure inside the analysis chamber was about 2 × 10 9 mbar. HR-XPS spectra were recorded using a 400 μm spot size, 0.1 eV step size, and 50 eV pass energy (200 eV for survey). All spectra were charge-corrected to the C 1s adventitious carbon (284.8 eV). Subsurface layers were measured with a depth profile by argon ion etching (1000 eV) in between XPS measurements. The obtained XPS spectra were deconvoluted with CasaXPS v2.3 software.

Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-OES).
The materials were dispersed in 35 vol % HNO 3 overnight to dissolve the carbides. The samples were further diluted with 3 vol % HNO 3 with an amount depending on the expected metal content. All ICP-OES measurements were performed on a SPECTRO ARCOS measured against an external calibration, with a typical detection limit of 10 ppb.

Scanning Electron Microscopy (SEM).
Prior to analysis, the aluminum cylindrical sample holder was washed in isopropanol in an ultrasonic bath for approximately 2 min. An isopropanol based catalyst ink was drop-casted on the sample holder and positioned in a 25 mm working distance. The SEM measurements were executed on a Jeol JSM 6500F instrument at an acceleration voltage of 15 kV, coupled with an energy dispersed X-ray analysis detector (Ultradry, Thermo Scientific).

Transmission Electron Microscopy (TEM).
A dispersion of catalyst and isopropanol was drop-casted on a TEM grid with a holey carbon film on a copper 400 mesh (EM-resolutions). All materials were analyzed with a JEOL JEM1400plus TEM at a 120 kV acceleration voltage using a single-tilt specimen holder. The TEM was equipped with a TVIPS TemCam-F416R high-resolution camera based on a custom designed CMOS architecture. ImageJ was used to estimate the particle size distribution.

Electrochemical Measurements.
A Biologic VSP-300 potentiostat in combination with EC-Lab software was used for all electrochemical measurements. The uncompensated resistance (R u ) of the system (the resistance between the reference electrode (RE) and working electrode (WE)) was measured before each cyclic voltammetry (CV) and chronoamperometry (CA) measurement. R u was determined with potentiostatic electrochemical impedance spectroscopy at open-circuit potential, with a frequency range between 200 kHz and 0.1 Hz. The distance between the origin and the first line intersection on the Z Real -axes within the Nyquist plot represents R u and was extracted by manual data fitting. Subsequently, the EC-Lab build-in IR compensation allowed 85% R u compensation without adding to much distortion to the CV and CA results. Only for the CA experiments, the other 15% R u was compensated after the measurement by using eq 2.
A polyether ether ketone (PEEK) three-electrode cell design adapted from the Jaramillo group was used for all electrochemical experiments. 43 It consisted of two separate compartments that accommodate 5 mL of electrolyte and 3 mL of gas headspace. An additional plate was added to the overall cell design ( Figure S2), which fixated the WE. A leak-free Ag/AgCl micro reference electrode (Innovative Instruments, LF-1-45) was used for potential control, wherein all potentials were recalculated versus the reversible hydrogen electrode scale following eq 3.
A Pt foil (50 × 50 × 0.025 mm, 99.99%, Mateck) functioned as the anode and was rinsed with water and flame annealed before each experiment. A fresh sheet of membrane (Celgard 3401) was used for every run, thereby preventing accumulation of NH 3 . The WE was prepared by drop-casting 3 droplets of 10 μL of a freshly prepared catalyst ink (2 mg cat ml −1 , 950 μL 2propanol (98%, VWR) and 50 μL of Nafion 117-containing solution (5 wt %)) on a carbon paper disk (1 cm 2 , Toray carbon paper, Aesar) with a loading of 0.06 mg cm −2 and stored under vacuum once prepared. The WE was soaked in a fresh 1 M KOH (99.95%), 0.1 M KOH, 0.5 M Li 2 SO 4 , or 0.05 M H 2 SO 4 solution before it was fixated in the cell by a glassy carbon plate (25 × 25 × 1 mm, HTW). The back of the glassy carbon was taped with a Cu strip (AT528, 10 mm width, RS Components) and connected to the potentiostat wires. The catholyte was saturated by purging N 2 or Ar for 30 min before each experiment. After cyclic voltammetry and chronoamperometric measurements, aliquots of both catholyte and anolyte were collected with a syringe and transferred to several test tubes for further quantification.
2.5. Minimizing Effects of Impurities. Feed gas contamination in the form of NH 3 and NO x in both high purity Ar and N 2 have been reported previously. 15,25 In order to remove residual contaminants, a certified commercial gas filter (Entegris GPUS35FHX) was installed upstream of the electrochemical cell (see Figure S3). The cell components were always acid cleaned with 10 vol % H 2 SO 4 for at least 1 h and rinsed with ultrapure water prior to each experiment. Syringes, needles, pipet tips, and sample tubes were also excessively washed with ultrapure water and dried under Ar flow before use. A microporous membrane (Celgard 3401) with a gas repellent coating was selected as a more suitable separator compared to the more commonly used Nafion membrane to avoid accumulation of ammonia contaminations as was reported previously. 15,44,45 A downstream acidified liquid trap is often used to measure volatile ammonia that could potentially be present in the effluent gas. As NH 3 dissolves very well in aqueous electrolytes (∼500 g/L), this suggests that low concentrations of NH 3 readily dissolves in the used electrolyte. This means that an acid trap is often redundant and can potentially be an extra source of contamination. 15 Therefore, we did not incorporate a downstream acidified trap in the experimental design.
Precursors and catalysts containing nitrogen species are potential sources of impurities and should be avoided. 28,30 The selection criteria for our catalyst synthesis procedures was mainly motivated by minimizing the use of N-containing precursors. The Mo 2 C nanoparticles and Mo 2 C nanodots do not contain N-based materials for the preparation, while the use of N−C compounds was unavoidable for the synthesis of iron carbide nanomaterials. The latter motivated us to use a catalyst loading of 0.06 mg·cm −2 to minimize the effects of the N−C precursor during the electrochemical experiments. We used a method adopted from Chen et al. to monitor impurities in our materials, 26 such as catalyst powders, membranes, carbon paper, and Pt foil. Strategies to effectively remove impurities will be discussed in a future study. 46 Li-salts are notorious for containing trace levels of NO x − species as was previously reported by Li et al. 47 Therefore, Li 2 SO 4 is suspected of having these labile N-species and the suggested thermal annealing step was implemented to remove trace impurities. For the annealing step, the as received Li 2 SO 4 (99.5%) was transferred to a tubular furnace and thermally annealed at 800°C for 4 h in Ar with a heating rate of 10°C· min −1 before preparing a solution. 2.6. Ammonia and Nitrite Quantification. Ammonia was quantified by the Berthelot reaction. 48 In a routine analysis, a volume of 1.  . X-ray diffractions patterns of (a) Mo 2 C NS (green) and Mo 2 C NP (blue) with the α-Mo 2 C (PDF 04-003-0962, black) and MoO 2 (PDF 04-013-3645, gray) reference patterns. The patterns in (b) and (c) represent Fe 3 C (purple) and Fe 5 C 2 (red) with the corresponding θ-Fe 3 C (PDF 00-035-0772, black) and χ-Fe 5 C 2 (PDF 01-080-4102, black) reference patterns. 35, and 74.2°), and Fe 2 O 3 (38.6°) are inevitable by-products of the carburization process. 41,50 Also, small fractions of Fe 3 O 4 (41.4°and 74°) were identified in Figure 1c after the thermal decomposition of Fe(CO) 5 , 41 while the multiplet between 49 and 55°is very typical for χ-Fe 5 C 2 (PDF 01-080-4102). The average crystallite size was calculated with the Scherrer equation (eq 1) and summarized in Table S1.
The fitted Mossbauer spectrum of the Fe 3 C sample ( Figure  2a) shows a sextuplet with an isomer shift (IS) of 0.19 mm s −1 , and a hyperfine field of 20.8 T. θ-Fe 3 C is identified as the major spectral contributor (67%). 51 Additionally, a metallic Fe sextuplet (IS = −0.004 mm s −1 , 33 T) was clearly observed and is in agreement with the sharp peak at 53.3°in the Fe 3 C diffractogram. A doublet indicates a quadrupole peak splitting, which means the absence of magnetic field spin coupling. This indicates the presence of (super)paramagnetic Fe 3+ nanostructures. It is difficult to allocate the specific Fe 3+ phase, as multiple subdoublets can be superimposed in one doublet. 52 However, the low intensity XRD peaks of Fe 2 Figure 2b, which were attributed to the three iron lattice sites in the Fe 5 C 2 crystal structure. 53,54 Fe 3 O 4 has a small spectral contribution located in the outer spectrum with an octahedral (IS = 0.28 mm s −1 , 49.1 T) and a tetrahedral site (IS = 0.71 mm s −1 , 46 T). 55 Fe 2 O 3 was not identified in the Fe 5 C 2 diffractrogram, which again suggests that the doublet is nanostructured Fe 3 O 4 . In conclusion, the Mossbauer data confirms the synthesis of the intended Fe 3 C and Fe 5 C 2 compounds with limited amounts of iron and iron oxide species. The remainder of the Mossbauer data is summarized in Table S2.
Mossbauer spectroscopy and XRD give information about the bulk phase of the material. XPS is surface sensitive and gives information regarding the surface phase and composition. The Mo 2 C XPS survey scans ( Figure S6a,b) contain peaks of Mo 3d, Mo 3p, C 1s and O 1s, from which the high-resolution scans of the Mo 3d photoelectrons (Figure 3a,b) were deconvoluted to identify different Mo oxidation states. The Mo 3d orbital has a spin−orbit Mo 3d 5/2 -Mo 3d 3/2 doublet with a 3/2 peak intensity ratio that is separated by a binding energy of 3.15 eV. The full width at half-maximum (FWHM) was kept constant for each doublet during the deconvolution process. It is often ambiguous to assign an oxidation state to Mo 2 C; therefore, it is often denoted in an aggregated term as Mo 0−3+ . 56 The presence of Mo 2 C surface bonded species was confirmed by the small Mo 3d 5/2 peaks at 229.3 and 228.2 eV for both Mo 2 C NS and Mo 2 C NP, respectively. 34,57 Other peaks at binding energies 232.5 and 235.5 eV for both Mocarbide materials are identified as MoO 3 and must be solely present in the thin surface layers as MoO 3 was not identified in the diffractograms. These spontaneously formed metal oxide surface layers are inevitable due to exposure to ambient air. Post-mortem XPS analyses confirmed that the majority of the surface layer was Mo 2 C. 23,58 This suggests that the Mo-oxide species are reduced during electrochemical reduction. Moreover, it is expected that the trans-passive Mo-oxide layers are  not stable in alkaline conditions and form soluble MoO 4 2− even at moderate reduction potentials. 59,60 The Fe 2p 3/2 peak was used to identify different Fe oxidation states. The broad peak between 714 and 709 eV contains a complex convolution of multiple subpentuplets of Fe 2 O 3 , Fe 3 O 4 , and FeOOH species, which all overlap in this region. We fitted one pentuplet as a general Fe-oxide term as indicated in Figure 3c,d by taking XPS reference data such as FWHM, relative peak area, and binding energies from Biesinger et al. 61 For iron carbide, the majority of the surface is covered with a thin Fe-oxide layer. According to the Pourbaix diagram for Fe, this oxide-layer is reduced by applying mild reduction potentials. 62 The presence of a single peak at 708.4 eV for Fe 3 C and 708.6 eV for Fe 5 C 2 is identified as the Fe carbide phase. Only Fe 5 C 2 has an additional sharp metallic Fe peak at 707.2 eV. The low signal-to-noise ratio for the Fe 5 C 2 Fe 2p spectra indicates a low Fe quantity (<1 at%), which is also reflected in a low intensity Fe oxide peak in the O 1s spectra ( Figure S7e). From a depth profiling test ( Figure S8), it becomes clear that the Fe 2p signal increases with a longer etching time, while the intensities of the O 1s and N 1s spectra decreases. This indicates that the top surface layer is covered with adventitious species due to atmospheric exposure.
The elemental Mo and Fe content in all metal carbides were analyzed by ICP-OES and are summarized in Table S3. The ICP-OES results revealed that the bulk concentrations of Mo and Fe are significantly higher with respect to the surface concentrations estimated by XPS. This suggests that the surface adsorption of advantageous species by air exposure is not only observed for Fe 5 C 2 but also for the other metal carbides.
The Mo 2 C NS are clearly visible in Figure 4a,e and confirm a successful synthesis. TEM imaging ( Figure S9a) reveals that a relatively large proportion of the sample consists of undecorated carbon nanosheets. This explains why the majority of the surface composition, analyzed by XPS, is predominantly carbon (Table S4). The existence of Mo 2 C nanodots (<20 nm), as proposed by Wang and coworkers, was not observed in our TEM analysis. 23 Despite the magnification limitations of the low-resolution TEM, distinguished nanoparticles up to 5−10 nm were detectable in other metal carbide samples, indicating that Mo 2 C nanodots of the order 10−20 nm should be visible. In Figure 4e and Figure S9b, regions with a higher contrast indicate a layer of aggregated Mo 2 C, with an average crystallite size (D XRD ) of 35 nm. The SEM−EDX results (Figures S10 and S11) support this observation and show that the carbon sheet is indeed covered with a nanocrystalline layer of Mo 2 C.
The successful synthesis of Mo 2 C NP on a carbon support was confirmed by TEM (Figure 4f and Figure S12). The particle size, D TEM , was distributed between 10 and 50 nm, which is in agreement with the average crystallite size (D XRD = 21 nm). The SEM images in Figure 4b and Figure S13 show a mesoporous morphology with a large surface area. The overall Mo 2 C surface distribution is homogeneous as was confirmed by the SEM−EDX mapping ( Figure S14). The carbon precursor in combination with the inert SiO 2 nanoparticles stimulates the spherical growth of nanosized Fe 3 C particles and prevents it from forming larger aggregates. Most Fe 3 C particles were between 40 and 60 nm. The Fe 3 C sample contained nanosized hollow features as visible in Figure 4c,g and confirmed the successful removal of SiO 2 during the 1 M KOH treatment. The absence of the SiO 2 nanoparticles after the treatment was further supported by Fourier transform infrared spectroscopy ( Figure S15). Small and isolated Fe 5 C 2 spherical nanoparticles with a narrow size distribution are observed in Figure 4d and Figure  S16 (D TEM is 5−35 nm and D XRD is 11 nm). This highlights that PVP successfully stabilizes the nanoparticles from agglomeration during the carbothermal reduction of Fe(CO) 5 . The material has a microporous structure ( Figure S17b) with a high surface area because of the polymeric nature of the support. In contrast to the low Fe content measured in the first ∼10 nm-thick surface layer, well distributed and significant Fe concentrations were detected in the bulk surface layers (∼1 μm) by SEM−EDX mapping ( Figure S18), which supports the XPS depth profiling and ICP-OES results ( Figure S8).

Electrochemical Characterization.
The currentpotential (I−V) relationship of each material was investigated by executing multiple CV cycles at a scan rate of 20 mV·s −1 in a N 2 presaturated electrolyte to estimate the onset potential and an expected potential window for the NRR. A possible pH dependency on the Mo 2 C activity of the NRR was investigated by executing CV in 0.05 M H 2 SO 4 (pH = 1), 0.5 M Li 2 SO 4 (pH = 8.3), 0.1 M KOH (pH 13), and 1 M KOH (pH 14). The stability of Fe-carbides in acidic-to-neutral conditions is low as the material tends to dissolve. 63 Therefore, we decided to only use 0.1 M and 1 M KOH for the evaluation of the Fecarbides. Mo 2 C is generally stable in both acidic and alkaline environments, allowing CV measurements in all electrolytes. 58,64 The uncompensated resistance (R u ), measured with open-circuit electrochemical impedance spectroscopy was consistent for each material tested and ranged between 25 and 30 Ω for 0.1 M KOH, 3 and 4 Ω for 1 M KOH, 12 and 13 Ω for 0.5 M Li 2 SO 4 , and 24 Ω for 0.05 M H 2 SO 4 ( Figure S19). These quantities for R u are below the acceptable range of the R u compensator used for all electrochemical measurements. Figure 5 shows that all metal carbide I−V relationships in acidic, neutral, and alkaline conditions display an increase in current density at increasingly more negative potentials, characteristic for an HER I−V profile. Other distinctive reduction peaks that might be identified as the NRR were not observed in the voltammetry measurements. In addition to this, there was no indication of a metal oxide reduction peak within the examined potential window, which suggests that the metal oxide surface layer is removed immediately. Mo 2 C NP reaches higher current densities compared to Mo 2 C NS at all pH values, which could be explained by a larger electrochemical surface area due to the mesoporous structure of Mo 2 C NP. Another explanation might be the higher Mo 2 C loading content in Mo 2 C NP, since the ICP-OES analysis resulted in a higher concentration of elemental Mo in Mo 2 C NP.
The onset potential is used to indicate the minimum activation potential of a redox reaction in cyclic voltammograms. 65 A theoretical approximation of the NRR onset potential in alkaline media can be calculated using the equilibrium potential and the activation overpotential. Firstprinciples density functional theory calculations suggest that the minimum overpotential for the NRR is approximately −0.4 V, due to scaling relationships between intermediates. 2,66 The NRR equilibrium potential was calculated by equilibrium thermodynamics and is 0.054 V vs RHE, which results in a minimum required onset potential (E onset, NRR ) of −0.35 V vs RHE. Here, we estimated the experimental E onset for different pH values by plotting the first derivative of the voltammogram (dj/dE) versus the applied potential ( Figure S20). The lift-off point where the slope of the dj/dE curve starts to increase is set as the E onset .
For 0.1 M and 1 M KOH, we do not see clear evidence for an alkaline pH effect for Mo 2 C materials as both Mo 2 C NP and NS have a similar I−V curve at both KOH concentrations. The onset potential for Mo 2 C NP of −0.11 V vs RHE is similar for both electrolytes and is in agreement with earlier observations. 64 Moreover, the onset potentials for Fe 3 C and  Figure 5c for Mo 2 C in 0.5 M Li 2 SO 4 is remarkably different showing a more negative onset potential of −0.28 V vs RHE and −0.32 V vs RHE for Mo 2 C NP and NS, respectively. As a consequence, the activation overpotential at −10 mA·cm −2 for Mo 2 C NP is −0.25 V lower than in alkaline conditions, which can be related to the low availability of either protons or hydroxide ions. At acidic conditions (pH = 1), Mo 2 C NP displays a similar I−V relationship with respect to alkaline conditions, which highlights the unique properties of Mo 2 C showing similar catalytic activity in both acidic and alkaline conditions. 64 However, the onset potential is slightly more negative (−0.17 V vs RHE), indicating that the catalyst is more active in alkaline conditions. E onset for Mo 2 C is above the theoretically estimated threshold in both acidic and alkaline pH, where the current density E onset,NRR is roughly −4 mA·cm −2 for Mo 2 C NP. From this analysis, it is unlikely that the NRR is a dominant contributor to the I−V profile of Mo 2 C because the HER kinetics are more facile in these conditions. Interestingly, E onset for Mo 2 C in 0.5 M Li 2 SO 4 is below E onset,NRR and suggests that operating at near-neutral conditions might be ideal for the NRR. It is important to note that Cheng and coworkers reported high NH 3 yields with Mo 2 C NS using the same electrolyte. 23 For Fe 3 C and Fe 5 C 2 , the majority of the I−V profile exceeds E onset,NRR , suggesting that both iron carbides might be promising catalysts for the NRR.
The NRR activity of the metal carbides was qualitatively screened by measuring the ammonia concentration after executing 40 scans of cyclic voltammetry with a scan rate of 20 mV s −1 . The results are summarized in Figure S21 and show that NH 3 concentrations for Mo 2 C in acidic-neutral pH is close to the detection limit <30 ppb, while levels up to 100 ppb were observed in alkaline conditions. Operating at alkaline conditions is therefore more beneficial for studying the NRR, and subsequently, the main electrochemical experiments herein were performed in alkaline conditions.
To ultimately verify Mo-and Fe-carbides as conceivable NRR catalysts, a series of 2 h chronoamperometry (CA) measurements were performed at five different potentials in a N 2 -saturated 0.1 M KOH electrolyte. The current densities for the metal carbides are stable in alkaline conditions as illustrated in Figure S22. As a comparison, two additional CA measurements in Ar-saturated electrolytes were performed with Mo 2 C, which gave slightly higher current densities for two potentials. The difference in current density is however rather small and can be caused by slight variations in the experiments. This observation was also made elsewhere, 28 questioning the

ACS Catalysis pubs.acs.org/acscatalysis
Research Article reliability of N 2 vs Ar voltammetry and CA experiments as an initial indicator for successful dinitrogen reduction.

NRR Measurements.
CA measurements were used to further assess the activity toward the NRR. After each CA, aliquots of both catholyte and anolyte were taken from the cell for further quantification of NH 3 and NO 2 − . The amount of quantified NH 3 after each experiment varied between 37 and 123 ppb with no particular trend linking ammonia concentration and applied potential over time. A 2 h open circuit potential (OCV) test with a N 2 -saturated electrolyte was used to obtain insights on the amount of impurities coming from either the feed gas stream or surface adsorbed species inside the cell. The OCV results for Mo 2 C reveal a similar NH 3 concentration as obtained with the chronoamperometry experiments. The impact of feed gas impurities can be excluded due to the installed certified gas filter (<100 ppt) in front of the cell. It is more likely that adsorbed NH 3 in the cell components is released during the OCV experiments and inevitably during the NRR measurements. Long term CA experiments with an Ar-saturated electrolyte are useful to study the possible release of N-impurities from the catalyst and other cell components exposed to the electrolyte under electrochemical conditions. For Mo 2 C, three CA experiments with Ar-saturated electrolyte at −0.20, −0.31, and −0.44 V vs RHE resulted in a somewhat lower NH 3 content (80, 53, and 60 ppb) compared to experiments with N 2 -saturated electrolytes. Again, it is deemed unlikely that purified Ar (and N 2 ) introduces feed gas impurities. Therefore, this observation suggests that the Ar gas flowing through the electrolyte stripped a small part of the dissolved NH 3 from the electrolyte. Nevertheless, both the N 2 OCV and Ar CA experiments indicate that the majority of the quantified NH 3 is not from the NRR but originates from contaminations. Additional control experiments with 15 N 2 -labeled gas were not performed since the observed NH 3 concentrations were below or approximating the background level.
Small quantities of NO 2 − were detected after all CA experiments, suggesting that a part of the quantified NH 3 potentially stems from NO x reduction. Jiao and coworkers observed that the electrochemical reduction of NO x forms multiple N-products, such as ammonia, hydroxylamine, N 2 , and N 2 O depending on the transition metal. 67 Pt is more selective toward NH 3 , which was also supported by Koper and coworkers who made a similar observation for NO 3 − electroreduction on Pt. 68 Mo 2 C has similar noble metal-like properties as platinum; 69 therefore, it is reasonable to assume that NO x species are reduced to ammonia at the investigated potentials. Fe 3 C is also an efficient nitrate reduction electrocatalyst, where a previous study reported faradaic efficiencies (FE) higher than 90% to NH 3 at moderate reduction potentials. 70 Despite the thorough cleaning efforts for every part of the cell, a well-established background level of both NH 3 and NO 2 − was always observed after each experiment. We decided to analyze the removal efficiencies of our cleaning methods (elaborated in the caption of Figure S23) and found that NH 3 was sufficiently removed by simply rinsing with water. Surprisingly, significant quantities of released NO 2 − were detected that originated from the Celgard 3401 membrane, carbon paper, and Pt foil. This is a valuable observation, as two previous studies advised substituting the Nafion membrane with a microporous Celgard membrane to reduce NH 3 contaminations. 15,45 Our results indicate that NO 2 − is not only a surface adsorbed species but is also present in the inner membrane and carbon paper structure and is problematic to remove. Generally, the amount of released NO 2 − depends mainly on the exposed surface area, meaning that it can be lowered significantly by optimizing the cell design. Investigating the origin of the observed NO x impurities is out of the scope of the present work and will be addressed in an upcoming study. 46 3.4. Literature Comparison. Previous studies using Moand Fe-carbides as NRR electrocatalysts are shown in a comparative overview (Figure 7a), including our own observations. It becomes clear that both our Fe 3 C and Fe 5 C 2 quantified NH 3 yields are within the NH 3 background level. The Mo 2 C catalysts exceed this threshold slightly, but with a significant NO x background, it becomes unlikely that any nitrogen reduction to NH 3 occurred. The study of Cheng and coworkers outperformed our Mo 2 C NS, observing a 240 times higher NH 3 yield. 23 This motivated us to execute a direct comparison by increasing the catalyst loading to 3 mg·cm −2 and using 0.5 M Li 2 SO 4 . The chronoamperometry measurements were comparable, but our NH 3 concentrations were below 100 ppb and close to the earlier defined background level as displayed in Figure 6. This is additional proof that Mo 2 C cannot be perceived as a promising NRR catalyst.

ACS Catalysis pubs.acs.org/acscatalysis
Research Article The majority of the earlier published literature observed orders of magnitude higher yields and FEs compared to this work. Firstly, all the literature studies shown in the overview did not quantify or consider NO x as an influential factor on their measured NH 3 content. Secondly, the impurities in the feed gas stream were not removed by the installment of a certified gas filter. This is especially important when performing 15 N 2 -isotope labeled experiments as traces of 15 N-labeled impurities ( 15 NH 3 and 15 NO x ) have been identified in several 15 N 2 -gas bottles. 15,25,71,72 Before using Li-based electrolytes for NRR experiments, Li-salts must be thermally annealed at 800°C under inert conditions to remove trace levels (>1 ppm) of NO x − impurities. 47 We followed this procedure, while others, including Cheng et al., did not consider this extraneous source of impurities, and this might be one of the main factors contributing to their high NH 3 yields. 23,31,36 As final point, the Nafion membrane commonly applied in these studies is known for the uptake and release of NH 3 during electrochemical experiments. 15 Substituting the Nafion membrane with another membrane is not straightforward as we detected a significant amount of NO 2 − in the microporous membranes (Celgard 3401), but selecting a suitable treatment method is advised. 73 Control experiments become even more essential when catalysts have a high N-content, such as metal nitrides, Ndoped supports, or leftover NO x /NH 3 traces from the catalyst synthesis. Evidence was found that for catalysts with a high N content, such as VN and Nb 4 N 5 , the decomposition of the Natomic lattice in acidic media released significant amounts of NH 4 + during the initial stages of the electrochemical experiment. 28 Similar observations were also reported for Mo 2 N. 30 Additionally, several commercially available metal oxide powders, such as Fe 2 O 3 and Bi 2 O 3 , released a large amount of NO x impurities. This eventually led to the retraction of a study, as it was proven that the origin of observed NH 3 was from NO x reduction and not the NRR. 26,74 Therefore, we decided to analyze the N-content of all four materials by XPS and UV−Vis spectroscopy (method described in the SI). The N 1s spectra of Mo 2 C could not be identified because of overlapping peak features with the Mo 3p orbital. Nevertheless, the absence of N KLL Auger peaks in both Mo 2 C NS and NP XPS surveys ( Figure S6a,b) indicate that the N-content might be negligible. Figure S7c,f shows two distinct N 1s peaks for Fe 3 C (398.3 and 399.9 eV) and Fe 5 C 2 (398.9 and 400.1 eV), suggesting pyridinic N−C and pyrrolic N−C bonds from the precursor (4.5-dicyanoimidazole and PVP). 75 Nevertheless, the samples were exposed to air before XPS analysis, indicating that the peaks could be also from adventitious N species, such as −NH 2 , which have similar binding energies. 75−77 It is therefore challenging to assign these peaks to a specific Nfunctional group. From the spectrophotometric analysis ( Figure S25), directly performed after the material synthesis, it becomes clear that NH 3 impurities from an unidentifiable source were present in all catalysts (Mo 2 C NS = 8.9 μmol NH3 g cat −1 , Mo 2 C NP = 16.5 μmol NH3 g cat −1 , Fe 3 C = 21.9 μmol NH3 g cat ). This effect was suppressed by using a low catalyst loading (0.06 mg) for each experiment. In the case of the most contaminated sample, the expected release of NH 3 from 0.06 mg Fe 3 C is limited to a negligible 1.3 nmol. Nevertheless, the NRR measurements performed with 3 mg Mo 2 C NS did not result in an increase in the NH 3 concentration. It remains unlikely that impurities in the catalyst resulted in exceptionally high NH 3 yield reported by Cheng et al. 23 This suggests that other factors lead to their positive result.
The NH 3 partial current density, j NHd 3 , is a useful performance indicator, wherein cases with j NHd 3 smaller than 100 μA cm −1 are too low to be promising. From a back-of-theenvelope calculation, we estimated that the NH 3 concentration at j NHd 3 =100 μA cm −1 is in the 1 ppm order of magnitude range assuming typical parameters, such as A WE = 1 cm 2 , V catholyte = 20 mL, and t CP = 1 h. These levels of NH 3 can easily be reached when the earlier mentioned sources of contamination are not identified or even considered. This has implications on the reliability and usefulness of reporting the FE, wherein the focus should be initially on j NHd 3 or the NH 3 yield rate. From Figure 7b, it becomes clear that most literature studies did not exceed 100 μA cm −1 , while a FE > 20% was reported (see Table S4). Therefore, we suggest that future publications explicitly report the NH 3 partial current density as the main catalyst performance indicator.
Interestingly, the role of metal carbides is also under debate for hydrazine oxidation. 78 Fe−N−C catalysts are common  Table S4. used catalysts for this reaction and contain iron carbides because of the high temperature pyrolysis required for the synthesis. Early studies claimed that Fe 3 C plays an active role in the reaction, 79,80 while a recent study revealed the true role of Fe 3 C by executing a rigorous comparison study between Fe−N−C materials with different amounts of Fe 3 C. 78 This approach led to the conclusion that Fe 3 C is mostly inactive for hydrazine oxidation, and should be removed by nonoxidizing acid solutions. This is yet another example of how a rigorous and well-designed experimental procedure can aid in clarifying the activity of electrocatalysts for reactions in the nitrogen cycle.

CONCLUSIONS
Nanostructured molybdenum carbide and iron carbide were reported earlier as promising electrochemical nitrogen reduction catalysts. In this study, the NRR activity of both molybdenum and iron carbide materials were reassessed with the implementation of a strict experimental protocol that allowed us to reduce the effects of extraneous impurities to a bare minimum and identify false positives. The successful synthesis of nanostructured Mo 2 C, Fe 3 C, and Fe 5 C 2 was confirmed by X-ray diffraction, scanning and transmission electron microscopy, and X-ray photoelectron and Mossbauer spectroscopy. The current−potential relationship of the metal carbides is characteristic for the HER, where the current increases with increasing negative overpotential. Moreover, specific reduction peaks that could be related to the NRR were not identified. NH 3 quantification was done after 40 scans of cyclic voltammetry, where we indeed measured NH 3 (50−100 ppb) for both Mo 2 C and Fe 5 C 2 in alkaline conditions. To further assess the NRR catalytic activity of molybdenum and iron carbides, we performed a series of 2 h chronoamperometry measurements at different potentials in N 2 -saturated 0.1 M KOH. For Mo 2 C NP and NS, the NH 3 concentration was between 41 and 124 ppb, exceeding the NH 3 background level (84−88 ppb) for potentials at −0.2, −0.31, and −0.46 V vs RHE. We noticed that the yield earlier reported by Cheng et al. was considerable higher than measured with our Mo 2 C NS. 23 A direct comparison by performing chronoamperometry experiments with an increased loading (3 mg·cm −2 ) and 0.5 Li 2 SO 4 did not result in elevated NH 3 concentrations. This is additional proof that Mo 2 C cannot be conceived as a promising NRR catalyst. The NO x content after the NRR, Ar, and OCV blank tests revealed NO 2 − concentrations in the same order of magnitude (55−122 ppb). This implies that NH 3 arises from NO 2 − reduction and not from the NRR. These NO 2 − impurities originated from the Celgard membrane, since we found that the membrane, even after rinsing excessively with water, released a considerable amount of NO 2 − impurities (109 ± 31 ppb). This emphasizes the importance of NO x monitoring, which is often overlooked in the literature and might result in a false positive. The quantified NH 3 from the iron carbide catalysts did not exceed the NH 3 background level, indicating that these materials are not active for the NRR. With our experimental approach, we succeeded in establishing a minimized and reproducible background level that allowed us to critically assess promising NRR catalysts. We believe that our methods and detailed analysis will equip researchers entering the field with clear guidelines to perform NRR experiments in a more reliable manner. ■ ASSOCIATED CONTENT